The present invention relates to a system for controlling a vehicle having a steer angle control system and another vehicle control system, such as a 4WD control system or a suspension control system, for controlling a condition of the vehicle affecting a handling characteristic of the vehicle.
Recently, many publications have disclosed various 4WS control systems, 4WD control systems and suspension control systems.
Some examples of 4WS control are disclosed in U.S. Pat. No. 4,705,131, Japanese Patent Provisional Publication No. 60-229873, and Japanese Utility Model Provisional Publication No. 62-23773.
Some examples of 4WD control are disclosed in U.S. Pat. No. 4,757,870, No. 4,754,834, No. 4,773,500 and No. 4,776,424, and "Seigyo Riron no Ohyo to Jidosha no Seigyo" (Application of Control Theory and Control of Automobile), Symposium of Society of Automotive Engineers of Japan, p41, published in 1986. The 4WD system disclosed in the last-named Japanese document is mounted on a NISSAN CUE-X concept car. The 4WD system of that document is an electronically controlled torque split, full-time 4WD system having a hydraulic multiple-disc clutch in a drive path for front wheels. A controller controls the torque capacity of the multiple disc clutch by varying an applied clutch fluid pressure. The driving force distribution ratio of the front wheel driving force to the rear wheel driving force is approximately Tc: (T-Tc), where Tc is the torque capacity of the multiple disc clutch, and T is the transmission output torque. Therefore, the controller controls the torque capacity Tc to provide an optimum front wheel and rear wheel driving torque distribution in accordance with signals of wheel speed sensors and an accelerator position sensor.
The Mitsubishi Galant is an example of a vehicle combining the 4WS system and the 4WD system.
An example of suspension control is disclosed in "T12 NISSAN Sahbisu Shuho" (NISSAN Service Bulletin), pages C-7, C-9 and C-10, published by NISSAN Motor Co., Ltd. October, 1985.
Some of the above-described control systems are equipped with a fail-safe means for detecting a failure, and electronically or mechanically adjusting an abnormal system in which the failure is detected so as to maintain the safety of the vehicle.
However, conventional fail-safe control systems are unsatisfactory in that no consideration is given to the influence of a failure of one control system on another control system in the same vehicle. In general, a vehicle having a second vehicle control system such as a 4WD control system or a suspension control system in addition to a 4WS control system, is tuned to have optimum vehicle characteristics when both control systems are normal and functioning properly. Therefore, the vehicle stability becomes worse when one control system is brought to a stop because of its failure.